It is known that organic compounds can be catalytically dehydrogenated and converted into corresponding unsaturated or aromatic compounds.
Usually, heterogeneous catalysts are used here. In the case of heterogeneous catalysts, a distinction is made between pure metals (for example in the form of colloidal metals, metal sponges or metal blacks, metal powders or wires), metal compounds (e.g. metal oxides, sulfides or nitrides; metal glasses) and supported catalysts. Dehydrogenation catalysts usually used are supported catalysts. Supported catalysts substantially comprise catalyst support (support) and active component.
It is known that catalysts lose activity during the reaction.
This takes place, for example, due to deposition and degradation of compounds on the catalyst surface. This phenomenon referred to as coking by the person skilled in the art leads to blocking of reactive centers and hence to deactivation of the catalyst.
It is advantageous to design the reaction procedure so that the deactivation of the catalyst is reduced in the reaction itself.
This can be effected by intrinsic improvement of the catalyst system. This approach is followed, for example, in EP-A 0 155 649. There, the conversion of piperidine derivatives into pyridines is carried out over a Pd/Al2O3 catalyst which additionally comprises from 0.1 to 10% by weight of MgCl2. This process is distinguished by an improvement in the achievable catalyst lives.
Furthermore, the deactivation of the catalyst can be suppressed by improving the course of the reaction during the hydrogenation.
EP-A 1 291 081 describes a process for the preparation of pyrroles and pyridines from pyrrolidones and piperidines, respectively, in the presence of a supported noble metal catalyst, in which the course of the synthesis is influenced without changing the chemical composition of the catalyst. EP-A 1 291 081 teaches that the formulation of valeronitrile over a ZrO2-supported Pd/Pt catalyst can be suppressed by the addition of water to the reactant stream. The addition of water requires an additional outlay in terms of process engineering and energy.
In the oxidative dehydrogenation, the dehydrogenation reaction takes place in the presence of oxygen. The heat input required for the dehydrogenation is effected by the combustion of hydrogen directly during the dehydrogenation reaction instead of via external heat input. The dehydrogenation and the oxidation can take place over the same catalyst or different catalysts. The dehydrogenation and the oxidation can take place together in the same location or separately.
EP-A 0 323 115 discloses the dehydrogenation of C2-C30-paraffins in the presence of steam and in the presence of a single catalyst. This preferably comprises platinum, potassium and tin on alumina as a catalyst support. The oxygen is used in about 0.01 to two molar amounts, based on the paraffin. The dehydrogenation is carried out at a temperature of from 400 to 900° C.
U.S. Pat. No. 3,670,044 describes the dehydrogenation of alkanes, alkylalkanes and arylalkanes having 2 to 12 carbon atoms in the presence of a catalyst which preferably comprises platinum and optionally tin on a zinc aluminum spinel as a catalyst support. The oxygen is used in from 0.02 to 0.15 molar amounts, based on the hydrocarbon used. The dehydrogenation is carried out at a temperature of from 510 to 621° C.
U.S. Pat. No. 5,733,518 describes the dehydrogenation of C3-C10-alkanes in the presence of nickel catalysts. Dehydrogenation and oxidation take place over different catalysts at different points in the reactor. The dehydrogenation is effected over a catalyst consisting of sulfided nickel on a non acidic support, such as neutralized alumina and zeolite. The hydrogen oxidation is effected in the presence of a phosphate of the metals germanium, tin, lead, arsenic, antimony and bismuth as a catalyst with 5.05 mol % oxygen in the feed at a temperature of from 300 to 600° C.
WO-A 94/29021 describes catalysts whose catalyst supports substantially comprise a mixed oxide of magnesium and aluminum, and which furthermore comprise a noble metal of group VIII and further components. These catalyze the dehydrogenation of C2-C30-hydrocarbons with or without simultaneous oxidation of hydrogen at a temperature of from 400 to 700° C.